fix(planner): prevent chart containers from overflowing on small screens

Grid children default to min-width:auto, letting the Chart.js canvas
push the container wider than its grid track. Adding min-width:0 and
overflow:hidden constrains charts to their column width.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

web/src/padelnomics/static/css/planner.css

@@ -455,6 +455,8 @@
   border-radius: 18px;
   padding: 1rem;
   box-shadow: 0 1px 4px rgba(0,0,0,0.04);
+  min-width: 0;
+  overflow: hidden;
 }
 .chart-container__label {
   font-size: 11px;